Despite compelling evidence that low levels of high density lipoprotein cholesterol (HDL) are associated with an increased risk of coronary heart disease, little is known about the determinants of HDL levels. Prior studies indicate that physically active men have elevated levels of HDL-cholesterol and lower levels of low density lipoprotein (LDL) cholesterol and triglycerides than sedentary individuals. It seems probable that the effects of exercise are mediated by changes in lipolytic enzymes or nutrient patterns, but possible interactions between exercise habits and the quantity and quality of calories in active individuals' diets have not been examined. The present proposal will test the hypothesis that the increased HDL levels characteristic of endurance athletes are secondary to their total caloric intake or patterns of carbohydrate and fat consumption. We will provide diets of defined composition to free-living endurance athletes and separately evaluate the effect of exercise cessation, reduced caloric intake, fat and carbohydrate supplementation, and variation of the diet polyunsaturated:saturated fat (P:S) ratio on serum lipoproteins and the activity of muscle and fat lipoprotein lipase. Changes in HDL2/HDL3 distribution will be defined by both ultracentrifugation and selective polyanionic precipitation. Levels of the major HDL apolipoproteins, apo A-I and apo A-II will be monitored by radioimmunoassay. This research should permit identification of the factor or factors responsible for the unusual distribution of lipoproteins and apolipoproteins in active men and yield information on the determinants of HDL levels. Such information may prove useful in designing strategies for the prevention of coronary heart disease.